This invention relates generally to semiconductor diode lasers and, more particularly, to one-dimensional arrays of semiconductor diode lasers fabricated as monolithic structures. Single-element diode lasers are limited in power to outputs of the order of 30 milliwatts (mW), but arrays of diode lasers can be designed to provide output powers of hundreds of milliwatts. Such high power outputs are useful in optical communications systems, laser printers and other applications.
A survey of the state of the art of phaselocked laser arrays can be found in a paper entitled "Phase-Locked Arrays of Semiconductor Diode Lasers," by Dan Botez and Donald Ackley, IEEE Circuits and Devices Magazine, Vol. 2, No. 1, pp. 8-17, January 1986.
By way of general background, a semiconductor diode laser is a multilayered structure composed of different types of semiconductor materials, chemically doped with impurities to give them either an excess of electrons (n type) or an excess of electron vacancies or holes (p type). The basic structure of the semiconductor laser is that of a diode, having an n type layer, a p type layer, and an undoped active layer sandwiched between them. When the diode is forwardbiased in normal operation, electrons and holes recombine in the region of the active layer, and light is emitted. The layers on each side of the active layer usually have a lower index of refraction than the active layer, and function as cladding layers in a dielectric waveguide that confines the light in a direction perpendicular to the layers. Various techniques are usually employed to confine the light in a lateral direction as well, and crystal facets are located at opposite ends of the structure, to provide for repeated reflections of the light back and forth in a longitudinal direction in the structure. If the diode current is above a threshold value, lasing takes place and light is emitted from one of the facets, in a direction generally perpendicular to the emitting facet.
Various approaches have been used to confine the light in a lateral sense within a semiconductor laser, i.e. perpendicular to the direction of the emitted light and within the plane of the active layer. If a narrow electrical contact is employed to supply current to the device, the lasing action will be limited to a correspondingly narrow region, in a process generally referred to as "gain guiding." At high powers, gain-guided devices have strong instabilities and produce highly astigmatic, double-peaked beams. For most high-power semiconductor laser applications there is also a requirement for a diffraction-limited beam, i.e. one whose spatial spread is limited only by the diffraction of light, to a value roughly proportional to the wavelength of the emitted light divided by the width of the emitting source. Because of the requirement for a stable diffraction-limited beam, most research in the area has been directed to index-guided lasers. In these, various geometries are employed to introduce dielectric waveguide structures for confining the laser light in a lateral sense, i.e. perpendicular to the direction of light emission and generally in the same plane as the active layer.
Most semiconductor structures employed for lateral index guiding in laser arrays are known as positive-index guides, i.e. the refractive index is highest in regions aligned with the laser elements and falls to a lower value in regions between elements, thereby effectively trapping light within the laser elements. Another type of index guiding is referred to as negative-index guiding, or antiguiding, wherein the refractive index is lowest in the regions aligned with the laser elements and rises to a higher value between elements. Some of the light encountering the higher refractive index material will leak out of the lasing element regions; hence the term leaky-mode laser array is sometimes applied.
In general, an array of laser emitters can oscillate in one or more of multiple possible configurations, known as array modes. In what is usually considered to be the most desirable array mode, all of the emitters oscillate in phase. This is known as the fundamental or 0.degree.-phase-shift array mode, and it produces a far-field pattern in which most of the energy is concentrated in a single lobe, the width of which is limited, ideally, only by the diffraction of light. When adjacent laser emitters are 180.degree. out of phase, the array operates in the 180.degree.-phase-shift array mode, or out-of-phase array mode, and produces two relatively widely spaced lobes in the far-field distribution pattern. Multiple additional modes exist between these two extremes, depending on the phase alignment of the separate emitters, and in general there are N possible array modes for an N-element array. Many laser arrays operate in two or three array modes simultaneously and produce one or more beams that are typically two or three times wider than the diffraction limit.
The present invention addresses two closely related problems pertaining to the operation of laser arrays at high powers and with high beam quality. The first problem involves the continuing search for a laser array structure of higher brightness (power per unit angle), operating in a selected array mode and without sacrificing efficiency and compactness. One aspect of the present invention addresses this need and provides a high-power output laser array operating in either the fundamental (in-phase) array mode or the out-of-phase array mode.
The second major problem area pertains to the degree of coherence and uniformity of output intensities that can be obtained across a laser array. Prior to the invention, coupling between lasers was limited to "nearest-neighbor" coupling to only adjacent lasers. This provided only a limited degree of overall coherence, and a characteristic cosine-shaped near-field intensity distribution. As outlined in the following summary, one aspect of the present invention provides a semiconductor laser array with a high degree of device coherence and a practically uniform near-field intensity distribution from a laser array.